Process for the production of titanium dioxide

ABSTRACT

It is disclosed a process for the production of titanium dioxide comprising the following steps: (a) a titanium ore containing iron, preferably ilmenite, is reacted with an aqueous NH 4 F solution; (b) the aqueous suspension thus obtained is filtered with consequent separation of a sludge fraction, which contains ammonium fluoroferrates, and a filtrate fraction, which contains ammonium fluorotitanates; (c) the filtrate fraction thus obtained is subjected to an hydrolysis reaction; (d) the thus-obtained solid component is subjected to a thermal hydrolysis reaction. The plant and the reactors for performing the above process are also disclosed.

The present invention relates to chemical reactors and may be used inprocesses of fluoride processing of titaniferous stock materials, forexample, ilmenite concentrates, in the production of titanium dioxide.

Known in the art is a reactor facility made as a cascade of reactors andapparatus, comprising a heat exchanger, a pipeline system and controlvalves (see the book by S. M. Korsakov-Bogatkov “Chemical Reactors asObjects of Mathematical Simulation”, Moscow: “Khimiya”, 1967, pp. 64-69,Fig. III-18).

A disadvantage of these solutions is that they cannot be usedeffectively for realizing the fluoride technology of processingtitaniferous stock materials, for example, ilmenite concentrates, in theproduction of titanium dioxide, because of insufficient endurance of theequipment.

Also known is a reactor facility comprising a reactor communicated withsources of reagents, which reactor is communicated through an unloadingunit with apparatus for subsequent processing of reaction products,wherein the reactor, apparatus and parts of the facility are made of amaterial resistant to the effect of reactive materials contacting saidreactor, apparatus and parts (see the book by S. M. Korsakov-Bogatkov“Chemical Reactors as Objects of Mathematical Simulation”, Moscow,“Khimiya”, 1967, pp. 64-69, FIG. III19).

However, this technical solution cannot be effectively used either forrealizing the fluoride technology of processing titaniferous stockmaterials, for example, ilmenite concentrates, in the production oftitanium dioxide, because of insufficient endurance of the equipment.Solving the problem of providing chemical resistance of the facility iscomplicated not only by the aggressiveness of the working medium, butalso by the thermal regime of operation (in the order of 800-900° C.)required for obtaining quality product (titanium dioxide having a highdegree of whiteness).

The problem to solving which the proposed technical solution is directedis to provide a possibility for using a reactor plant for realizing thefluoride technology of processing titaniferous stock materials toproduce white and red pigments.

The technical result obtainable upon solving the posed problem isexpressed in higher reliability and operability of the reactor facilityunder the conditions of highly aggressive fluoride-containing materialsbeing employed in processing titaniferous stock materials, to producewhite and red pigments. Furthermore, a high completeness of utilizationof the stock materials along with a high yield and whiteness of theproduct are ensured.

Furthermore, as compared with the “chlorine” technology of processing,the technological process is simplified (the necessity in the steps ofmetallurgical processing, producing chlorine, and other power-demandingoperations is obviated); while, as compared with the “sulfate”technology of processing, an appreciably higher quality of the productand the absence of wastes are ensured (the amount of wastes in the“sulfate” technology exceeding essentially the yield of the finishedproduct: the production of 1 ton of titanium dioxide involves 3 tons ofiron sulfide and 4 cubic meters of hydrolytic sulfuric acid which isvery difficult to regenerate).

The posed problem is solved by that a reactor facility comprising areactor communicated with sources of reagents, which reactor iscommunicated through an unloading unit with apparatus for subsequentprocessing of reaction products, wherein the reactor, apparatus andparts of the facility are made of a material resistant to the effect ofreactive materials contacting said reactor, apparatus and parts ischaracterized in that as the sources of reagents use is made of a binfor a solid titaniferous material, for example, ilmenite, and a sourceof ammonium fluoride; the unloading unit comprises filtrate, sludge andgas outlets, the gas outlet of the reactor being communicated with afeeder of ammonia, the filtrate outlet of the reactor is communicatedwith a first filter whose filtrate outlet is communicated with a secondfilter whose filtrate outlet is communicated with the interior of ahydrolysis reactor whose outlet in its turn is communicated with a thirdfilter whose sludge outlet is communicated with a first dispersing dryerwhose sludge outlet is communicated with a loading unit of a firstthermal hydrolysis reactor whose outlet is communicated with a containerfor storing white pigment, wherein the gas outlets of a second filter ofthe first dispersing dryer, of the third filter and of the first thermalhydrolysis reactor are communicated with the source of ammoniumfluoride; furthermore, the feeder of ammonia is communicated with thesecond filter and with the interior of the hydrolysis reactor, thesource of ammonium fluoride being additionally communicated with theinterior of the hydrolysis reactor; furthermore, the sludge outlets ofthe reactor and of the first filter are communicated with a seconddispersing dryer whose sludge outlet is communicated with the interiorof a second thermal hydrolysis reactor whose outlet is communicated witha container for storing red pigment, the gas outlets of the seconddispersing dryer and of the second thermal hydrolysis reactor beingcommunicated with the source of ammonium fluoride; furthermore, theinterior of the first thermal hydrolysis reactor and the interior of thesecond thermal hydrolysis reactor are communicated with a source ofsteam via steam pipes. Furthermore, the source of ammonium fluoridecomprises a storage for ammonium fluoride, communicated with the feederof ammonium fluoride via an evaporator whose vapor outlet iscommunicated via a condenser with a container for storing ammonia water,as the outlets of the source of ammonium fluoride use being made of theoutlets of the feeder of ammonium fluoride, and as the inlets of thesource of ammonium fluoride use being made of the inlets of the storagefor ammonium fluoride. Furthermore, the feeder of ammonium fluoride iscommunicated via a heater with a feeder of ammonium. Furthermore, thesludge outlet of the second tilter is communicated with the inlet of thefirst filter. Furthermore, the interior of the hydrolysis reactor iscommunicated with a source of modifying agents.

A comparative analysis of the features of the claimed solution with thefeatures of the prototype and of the analogs testifies to the conformityof the claimed solution with the criterion of “novelty”.

The features of the distinctive clause of the set of claims providesolution of the following functional problems:

The features “as the sources of reagents use is made of a bin for asolid titaniferous material, for example, ilmenite, and a source ofammonium fluoride” provide realization of a first step of the technologyof fluoride processing of titaniferous stock materials: “stripping” thestarting product (its conversion into a physicochemical state providingthe feasibility of the subsequent processing step).

The features “the unloading unit comprises filtrate, sludge and gasoutlets” provide switching over (transferring) the reaction products tocorresponding “technological chains”, the latter unit (together with thefeature “the gas outlet of the reactor being communicated with a feederof ammonia”) ruling out losses of ammonia (which is a waste in the firststep of processing, but at the same time is one of the re agents used inthe subsequent steps).

The features “the filtrate outlet of the reactor is communicated with afirst filter whose filtrate outlet is communicated with a second filterwhose filtrate outlet is communicated with the interior of a hydrolysisreactor” describe “a line for fine purification of the filtrate” of thetechnological chain of obtaining white pigment from iron compounds,i.e., provide removal of those admixtures whose presence in the finishedproduct would not allow obtaining a high degree of whiteness of thepigment.

The features indicating that the outlet of the hydrolysis reactor iscommunicated with a third filter whose sludge outlet is communicatedwith a first dispersing dryer whose sludge outlet is communicated with aloading unit of a first thermal hydrolysis reactor” describe “a unit fordehydrating” ammonium oxofluorotitarnate in the technological chain ofobtaining white pigment, which provides preparing thereof to thermalhydrolysis.

The presence of a first thermal hydrolysis reactor provides (togetherwith the feature regulating coupling the reactor interior to the sourceof steam) the possibility of processing ammonium oxofluorotitanate intowhite pigment and transferring it to the container for storing whitepigment.

The features “the gas outlets of a second filter of the first dispersingdryer, of the third filter and of the first thermal hydrolysis reactorare communicated with the source of ammonium fluoride” providereiterated use of this reagent, reducing its consumption, and therebyimprove the technical and economic characteristics of the process forproducing white pigment.

The features “the feeder of ammonia is communicated with the secondfilter and with the interior of the hydrolysis reactor” provideprecipitating iron-containing components from solution of ammoniumoxofluorotitanate and thereby its complete separation upon filtering.

The features “the source of ammonium fluoride being additionallycommunicated with the interior of the hydrolysis reactor” providehydrolysis of ammonium hexafluorotitanate.

The features “the sludge outlets of the reactor and of the first filterare communicated with a second dispersing dryer whose sludge outlet iscommunicated with the interior of a second thermal hydrolysis reactorwhose outlet is communicated with a container for storing red pigment”make it possible to prepare material to the thermal hydrolysis ofammonium hexafluoroferrate in the technological chain of producing redpigment and to carry out the process of thermal hydrolysis (upon steamsupply), thereby ensuring complete utilization of the stock materialowing to broadening the range of obtained products and making theutilization of the reagents more complete (in joint “operation” of thefeature with the features “the gas outlets of the second dispersingdryer and of the second thermal hydrolysis reactor being communicatedwith the source of ammonium fluoride”).

The features of the second claim describe a possible variant ofstructural embodiment of a source of ammonium fluoride; moreover, theymake it possible to utilize excess water containing ammonia and toobtain additional products therefrom.

The features of the third claim make it possible to compensate for theloss of ammonia as it is gradually consumed (removed with water vapors).

The features of the fourth claim make it possible to rule out losses ofthe starting material suitable for producing red pigment.

The features of the fifth claim make it possible to “control” thequality of the obtained product.

FIG. 1 shows a schematic diagram of the facility; figure Ibis shows analternative version of the facility wherein the first thermal hydrolysisreactor consists of two series-connected reactor blocks (43 and 44),which are also separately represented by FIGS. 3 and 4, respectively;FIG. 2 is a sectional view of reactor 1; FIG. 3 is a sectional view ofthe first stage of the first thermal hydrolysis reactor; FIG. 4 is asectional view of the second stage of the first thermal hydrolysisreactor.

As it will be apparent from the following description, the main objectof the present invention is represented by a process for the productionof titanium dioxide comprising the following steps:

-   (a) a titanium ore containing iron, preferably ilmenite, is reacted    with an aqueous NH₄F solution;-   (b) the aqueous suspension thus obtained is filtered with consequent    separation of a sludge fraction, which contains ammonium    fluoroferrates, and a filtrate fraction, which contains ammonium    fluorotitanates;-   (c) the filtrate fraction thus obtained is subjected to an    hydrolysis reaction;-   (d) the thus-obtained solid component is subjected to a thermal    hydrolysis reaction.

Step (a) is preferably performed at 100-120° C., at a pressure of about1-2 bar and at a pH of about 6.5-7.0; the aqueous NH₄F solution normallyhas a concentration of 30-60% by weight, preferably about 45%.

According to a preferred embodiment of the invention, the thermalhydrolysis reaction (c) is performed in two reactors; the first reactoris maintained at a temperature of up to 300-350° C. whereas the secondreactor is maintained at a temperature of up to 800-900° C. The body ofthe first and/or second reactor is preferably made of a chromium-nickelalloy; the internal surface of the first reactor is preferably made ofmagnesium or a graphite-reinforced polymer or vitreous carbon whereasthe internal surface of the second reactor is preferably made of silica.

The sludge fraction of step (b) may also be subjected to a thermalhydrolysis reaction which is preferably performed at a temperature of upto 300-350° C.

A further object of the invention is represented by the plant forperforming the above process, as for instance in the form represented byFIGS. 1 and 1bis; additional objects of the invention are alsorepresented by the reactors for performing the reaction (a) and thethermal hydrolysis reaction (d), as for instance in the formsrepresented by FIGS. 2, 3 and 4.

Shown in the drawings are a reactor 1 communicated with a bin 2 and asource 3 of ammonium fluoride. Also shown in the drawings are filtrateoutlet 4, sludge outlet 5 and gas outlet 6 of the reactor 1; a feeder ofammonia 7, a first filter 8 with a filtrate outlet 9 and a sludge outlet10, a second filter 11 with a filtrate outlet 12 and a sludge outlet 13;a hydrolysis reactor 14 whose outlet 15 is communicated with a thirdfilter 16 whose sludge outlet 17 is communicated with a first dispersingdryer 18 whose sludge outlet 19 is communicated with a loading unit 20of a first thermal hydrolysis reactor 21 whose outlet is communicatedwith a container 22 far storing white pigment. The filtrate outlet 4 ofthe reactor 1 is communicated with the first filter 8, and its sludgeoutlet 5 is communicated with a second dispersing dryer 23. The gasoutlet 6 of the reactor 1 is communicated with the feeder 7 of ammonia.The filtrate outlet 9 of the first filter is communicated with thesecond filter 11, and its sludge outlet 13 is communicated with a seconddispersing dryer 23. The filtrate outlet 12 of the second filter iscommunicated with the hydrolysis reactor 14, and its sludge outlet 13 iscommunicated with the inlet of the first filter (with the filtrateoutlet 4 of the reactor 1). Also shown in the drawings are gas outlets24 of the second filter, of the first dispersing dryer, of the thirdfilter, of the first thermal hydrolysis reactor 25, which by means ofgas collecting mains 26 are communicated with a storage 27 of the source3 of ammonium fluoride; besides, the feeder of ammonia 7 is shown, whichby means of a gas main 28 is communicated with the second filter 11 andwith the interior of the hydrolysis reactor 14; a feeder 29 of thesource 3 of ammonium fluoride is communicated both with the interior ofthe hydrolysis reactor 14 and with the interior of the reactor 1, aswell as with a heater 30; also shown is a container 31 for storing redpigment, which is communicated with the outlet of the second thermalhydrolysis reactor 25; besides, source of steam 32 is shown, which iscommunicated with the interior of the first thermal hydrolysis reactorand with the interior of the second thermal hydrolysis reactor via steampipes 33. The source 3 of ammonium fluoride further comprises a storage27 of ammonium fluoride, communicated with the feeder 29 of ammoniumfluoride via an evaporator 34 whose steam outlet is communicated via acondenser 35 with a container 36 for storing ammonia water. The outletsof the feeder 29 of ammonium fluoride, made as pipelines 37, serve asthe outlets of the source 3 of ammonium fluoride, while the inlets ofthe storage 27 of ammonium fluoride, made as collecting gas mains 26,serve as the inlets of the source of ammonium fluoride. Further, thefeeder 29 of ammonium fluoride is communicated with the feeder 7 ofammonia via the heater 30. The interior of the hydrolysis reactor isfurther communicated with a source 38 of modifying agents.

Since the claimed reactor facility is intended for realizing thefluoride technology of processing titaniferous stock materials: all theunits thereof: the reactor, thermal hydrolysis reactors, hydrolysisreactors, filters, disintegrator dryers, pipelines and other memberscontacting aggressive fluorine-containing reagents and reactionmaterials are made of a material resistant to the effect of the reactionmaterials contacting them (within the working temperature ranges).

It is expedient to use a vertical (top-down) integration of thefacility, wherein the apparatus providing the first technological stepsare arranged above the apparatus providing subsequent technologicalsteps. This will allow easy shifting of sludge-like reaction materialsalong the technological chain by gravity.

Reactor 1 employed in the facility of the invention (see FIG. 2) is areactor having a conventional structural layout: comprising a stationarysealed cylindrical body having a vertical axis: in the interior of whicha rotary shaft with stirrers 39 provided with a rotary speed governor isdisposed. The reactor body has a cover through which branch pipes arepassed: a loading branch pipe 40 (communicated with the bin 2) and areagent branch pipe 41 (communicated with the source 3 of ammoniumfluoride), as well as the filtrate outlet 4 and the gas outlet 6 of thereactor. The sludge outlet 5 of the reactor is located in the reactorbottom. The reactor is rated for temperatures of 100-120° C. Theprescribed temperature regime is provided by a heat supply unit 42 madeas a jacket (an additional shell) arranged on the lower portion of thebody and bottom of the reactor and coupled to a source of heat carrier(not shown in the drawings). The reactor body is made of a structuralmaterial, namely, of a chemically stable chromium-nickel alloy Type06XH28M

T, and its internal surface contacting the reagents, as well as otherparts and units disposed in the interior of the reactor body, are madeof magnesium or a graphite-reinforced polymer or vitreous carbon, or areprovided with a protective coating made of the above-said materials.

The first filter 8 and the second filter 11 do not differ in theirdesign from conventional apparatus having a similar purpose (except forthe material from which they are manufactured and tight sealing of theworking space). Said filters differ from each other only in the workingparameters of the filtering units (the second filter 11 provides a finerfiltration and, the second filter is additionally coupled to the feeder7 of ammonia and provided with the gas outlet 24).

The hydrolysis reactor 14 does not differ from conventional apparatushaving a similar purpose (except for the material from which it ismanufactured, tight sealing of the working space, and the number andpurpose of the units for the inlet and outlet of the reaction materialsand products).

The third filter 16 does not differ in its design from conventionalapparatus having a similar purpose (except for the material from whichit is manufactured, tight sealing of the working space; and theprovision of the gas outlet 24) built around centrifuges, this beingdictated by the consistence of the material fed to its inlet.

The first and second dispersing dryers 18 and 23 are similar in design(differing only in their throughput capacity) and do not differ fromconventional apparatus having a similar purpose (except for the materialfrom which they are manufactured, tight sealing of the working space,and provision of the gas outlets 24).

The loading units 20 of the first thermal hydrolysis reactor 21 and ofthe second thermal hydrolysis reactor 25 are made as tightly sealedreservoirs interconnected by tight inclined ducts providing gravity feedof loose materials into the thermal hydrolysis reactors (their purposebeing to provide time-stabilized flow of the reaction material beingloaded). The first thermal hydrolysis reactor 21 and the second thermalhydrolysis reactor 25 differ from the reactor 1 by the structural layout(their longitudinal axis being disposed at an angle of up to 10° C. tothe horizontal) and by the cylindrical body rotating about this axisbeing mounted in stationary journals (constituting stationary end wallsof the body). Because of difficulties with selecting a material formanufacturing the internal surfaces of the reactor, which mustsimultaneously have a high chemical resistance to fluoride containingmaterials and preserve strength at high working temperatures (up to 900°C.), it is most expedient to carry out the process thermal hydrolysis intwo steps (the first step at temperatures lowered to 300-350° C. underconditions of maximum concentrations of the fluoride-containingcomponents, followed by treating the material obtained in the first step(wherein the concentration of the fluoride-containing components islowered by an order of magnitude and more) at a higher level oftemperatures of up to 900° C.). For this purpose it is possible to use astage of two series-connected reactor blocks 43 and 44 for thermalhydrolysis, having the same design (except for lining their interior).The body of the first of said blocks is made of a structural material,namely, of a chemically stable chromium-nickel alloy Type 06XH28M

T, and its internal surface contacting the reagents, as well as otherparts and units disposed in the interior of the reactor body, are madeof magnesium or a graphite-reinforced polymer or vitreous carbon, or areprovided with a protective coating made of the above-said materials.

The body of the second of said blocks is made of a structural material,namely, of a chemically stable chromium-nickel alloy Type 06XH28M

T, and its internal surface contacting the reagents is made of silica(pressed disperse quartz). Each of the reactor blocks of the stages (ofthe first and second thermal hydrolysis reactors 21 and 25) is coupledvia the steam pipe 33 to the source 32 of steam (made as a conventionalgenerator of superheated steam). Each of said blocks is also coupledwith the gas outlet 24 to the gas collecting main 26. Drives 45 forrotating the reactor bodies are made as electric motors with reducinggears whose output gears 46 are mounted with the possibility ofinteracting with a toothed rim 47 rigidly fixed on the cylindricalportion of the body of each of the reactor blocks. The stock material isloaded into the first reactor block 43, the finished product is unloadedfrom the second reactor block 44. The body of each of the reactor blocksbeing mobile, a heat supply unit 48 must provide non-contact heating.Therefore, in contradistinction to the reactor 1, it is expedient thatthe heat supply unit 48 should be of induction type (for instance,should comprise electromagnetic inductors mounted on annular framesencompassing the shell and ensuring non-contact high-frequency heatingof the external shell of the reactors). The design of the second thermalhydrolysis reactor 25 is similar.

The source 38 of modifying agents is made as a bin, sealed off from theambient medium and provided with a means for supplying modifying agents(a fine-dispersed mixture of salts of zinc, aluminum, zirconium,silicon) to the hydrolysis reactor 14 (made, e.g., as an inclined pipeproviding gravity feed of loose material).

Containers 22 and 31 for storing the final product (white and redpigments) and the container 36 for storing ammonia water are similar indesign (the difference being in the means for unloading the containersand also in the material: the surface of the container 22, whichcontacts the product, is made of a material which is either unoxidizableor gives colorless products of oxidation). The feeder 29 and the storage27 of ammonium fluoride are made as tightly sealed containers forstoring ammonium fluoride, provided with appropriate pumping means (notshown in the drawing). The feeder 7 of ammonia is made as a tightlyclosable container provided with conventional dispensing units, such asfilling nozzles made of a material resistant to the effect of ammonia.

The evaporator 34, condenser 35 and heater 30 are made as heat exchangeapparatus providing either heat supply to the liquids being pumpedtherethrough (the evaporator 34 and the heater 30) or removal of heatfrom the vapor-liquid flows being pumped therethrough (the condenser35).

The detachable parts of the reactor bodies, thermal hydrolysis reactors,other apparatus comprised in the facility, and the contact surfaces ofmovable connections are made tight with the help of seals (not shown inthe drawings) made of a sufficiently resilient, chemically stablematerial, preferably of a polymeric material based on carbon-reinforcedplastics or polypropylene, if the latter withstands the workingtemperatures of the reactor.

Besides, the facility comprises a set of conventional instrumentationequipment (not shown in the drawings) for controlling the workingconditions (temperature, volume of loading, acidity of the medium, andother working parameters).

The claimed facility operates in the following manner.

A batch of a titaniferous stock material, e.g. of ilmenite concentrate,whose basic component is ilmenite (FeTiO₃), is loaded into the interiorof the reactor 1 from the bin 2 through the loading branch pipe 40, andan aqueous solution of ammonium fluoride (NH4F) (with a large excess ofthe latter) is introduced into the interior of the reactor 1 through thereagent branch pipe 41 from the feeder 29 of the source 3 of ammoniumfluoride. The drive of the rotary shaft with stirrers 39 is switched on,providing continuous stirring of the reaction components, and the heatcarrier is fed to the heat supply unit 42. The external surface of thereactor 1, contacting the heat carrier, becomes heated and gives offheat to the interior of the reactor, bringing the temperature therein to90-110° C. Vapors of ammonia and water are vented through the gas outlet6. After the expiry of time which is determined, for example,empirically with taking into account temperature parameters,concentrations of the reagents, etc., for concentrates differing in thecontent of useful component, or by taking samples from the reactor andcarrying out their rapid analysis, the resulting liquid fractioncontaining a fine suspension of insoluble ammonium fluoroferrates insolution of ammonium fluorotitanates is removed from the reactor throughthe filtrate outlet 4).

Then a new batch of components is loaded into the reactor and the wholeprocedure is repeated. Since the process of stripping the titaniferousstock material is cyclic, it is expedient either to have severalreactors in operation or to use intermediate storage containers whosevolume allows ensuring time-constant volume of the stripped sockmaterial supply.

Introducing aqueous ammonium fluoride solution under the loaded volumeof the solid reaction component (ilmenite concentrate) will additionallypromotes intermixing of the reagents by the bubbles of evolving ammonia.

The rotary speed of the shaft with stirrers 39 is adjusted so that thestirring of the reaction components should proceed without needlessroiling of the forming liquid fraction (i.e., without transferring intosuspended state the incompletely reacted solid particles of the solidcomponent, having sufficiently large hydraulic size).

Since not only useful components but also ballast components are presentin the composition of ilmenite concentrate, ballast components (sludge)accumulate in the course of the reactor operation. Periodically, afterthe removal of the formed liquid fraction; sludge is removed from thereactor interior, the sludge outlet 5 opened for this purpose.

Further the suspension of insoluble ammonium fluoroferrates in solutionof ammonium ferrotitanates is fed to the first filter 8, wherein primaryseparation of the solution into a sludge fraction (containing ammoniumfluoroferrates) and a filtrate fraction (containing ammoniumfluorotitanates), and appropriate routing of said materials to thetechnological chain of producing red pigment or to the technologicalchain of producing white pigment, respectively, are carried out.

In the technological chain of obtaining white pigment, the filtratefraction (containing ammonium fluorotitanates) comes to the secondfilter 11, wherein a the second (finer) degree of purification iscarried out, feeding ammonia to the second filter (from the feeder 7 ofammonia) contributing to the coagulation and precipitation of ironsalts.

The sludge fraction is returned to the inlet of the first filter 8 , andthe filtrate fraction is fed to the hydrolysis reactor 14, wherein it iscontacted with the aqueous solution of ammonium fluoride (NH₄F) and themodifying additives supplied, respectively, from the source 3 ofammonium fluoride, the feeder 7 of ammonia, and the source 38 ofmodifying agents. As a result, a sludge (paste-like mass) of ammoniumoxofluorotitanate is obtained at the outlet 15 of the hydrolysis reactor14. This material is dehydrated by passing the aqueous solution ofammonium fluoride through the third filter 16 and finally drying andcomminuting it on the first dryer/disintegrator 18. Then, via theloading unit 20; the loose titanium oxofluorotitanate is passed throughthe reactor blocks 43 and 44 of the first thermal hydrolysis reactor,whereto superheated steam is supplied simultaneously, the temperature ofup to 300-350° C. being maintained in the reactor block 43 and thetemperature of up to 900° C. being maintained in the reactor block 44.

The material moves in the interior of the reactor blocks, because, asthe bodies of the reactor blocks rotate, particles of the solidcomponent roll over and slide down by gravity down the surface formed bythe particles of the material in the interior of the reactor block. Thissurface has the form of an inclined plane whose upper end is located onthe side toward which rotation is directed, and as soon as the particlesreach the level of the original dip surface, they roll down. Since thelongitudinal axis is inclined, the movement of the particles occurs notwithin the transverse plane of the shell, but has a vector directed fromthe inlet to the outlet. Therefore the superheated steam can all thetime be in contact with the “self-intermixing” particles of the solidcomponent. The operation of the heat supply unit 48 ensures theprescribed temperature regime of the reactor operation owing to thenon-contact heating of the external surface of the reactor units andheat transfer to the internal surface of the reactor interior, andsubsequent radiation of heat into the interior of the reactor block. Theheat is thus transferred to the particles of the solid component, whichare in contact with the interior of the reactor block, and thetemperature in the reactor interior becomes brought to 300-350° C. NH₄Fand HF formed in the course of the reaction of ammoniumoxofluorotitanate with superheated steam are vented together with watervapors through the gas outlet branch pipe 24. The solid component(containing TiO₂ and the remaining part of ammonium oxofluorotitanate(up to 10% of the initial amount) is transferred into the thermalhydrolysis reactor block 44. This block is rated for the temperatureregime of up to 800-900° C. and operates similarly to the one justdescribed above, but the initial product supplied thereinto is thematerial comprising TiO₂ and the remaining part of ammoniumoxofluorotitanate (up to 10% of the initial amount). As the solidcomponent moves along the lining made of pressed disperse quartz, itsmaterial enters into reaction with HF (evolving in the course of thereaction), giving silicon tetrafluoride (a volatile compound) which isremoved together with waste gases through the gas outlet 24.

The contact of the superheated steam fed to the reactor interior withthe remained part of unreacted ammonium oxofluorotitanate at atemperature of up to 800-900° C. leads to its entering completely intothe reaction. This provides obtaining at the outlet quality titaniumoxide (TiO₂). This titanium oxide is unloaded into the container 22 forstoring white pigment. During the operation of the facility, NH₄F and HFformed in the second filter 11, in the first dispersing dryer 18, in thethird filter 16. in the first thermal hydrolysis reactor 21, aredischarged through their gas outlets 24 together with water vapors intothe collecting gas mains 26 and further to the storage 27 of ammoniumfluoride. For restoring the concentration of ammonium fluoride, thematerial thus collected is subjected to evaporation in the evaporator34. The evaporating water vapors contain up to 2% of ammonia. Aftertheir condensation the resulting ammonia water is discharged into acontainer for storage thereof. The amount of ammonia in the feeder 7 ofammonia is replenished by discharging ammonia from the reactor 1 intofeeder 7. If this proves to be not sufficient, then, owing to theoperation of the heater 30, a corresponding portion of ammonium fluorideis subjected to decomposition (the appropriate portion of ammoniumfluoride being withdrawn from the pipeline communicating the feeder 29of ammonium fluoride and the hydrolysis reactor 14) to produce ammoniavapors which are also discharged into the feeder 7 of ammonia.

In the technological chain of producing red pigment, the sludge fraction(containing ammonium fluoroferrates) obtained at the sludge outlets 5and 10 of the reactor 1 and the first filter 8: respectively, isdehydrated and dried (by venting anmnonium fluoride together with watervapors), then this sludge fraction is comminuted on the seconddisintegrator dryer 23. After that loose ammonium fluoroferrate isloaded through the loading unit 20 into the second thermal hydrolysisreactor 25 and passed through the reactor blocks of thermal hydrolysis,whereto superheated steam is supplied simultaneously with similar regimeparameters (the temperature of 300-350° C. being maintained in the firstreactor block of thermal hydrolysis and the temperature of up to 900° C.being maintained in the second reactor block of thermal hydrolysis). Thefinished product (red pigment) is accumulated in the container 31.

1. A process for the production of titanium dioxide comprising thefollowing steps: (a) a titanium ore containing iron is reacted with anaqueous NH₄F solution; (b) the aqueous suspension thus obtained isfiltered with consequent separation of a sludge fraction and a filtratefraction; (c) the filtrate fraction thus obtained is subjected to anhydrolysis reaction; (d) the thus-obtained solid component is subjectedto a thermal hydrolysis reaction.
 2. A process according to claim 1,wherein the sludge fraction of step (b) contains ammoniumfluoroferrates.
 3. A process according to claim 1, wherein the filtratefraction of step (b) contains ammonium fluorotitanates.
 4. A processaccording to claim 1, wherein step (a) is performed at 100-120° C.
 5. Aprocess according to claim 1, wherein step (a) is performed at apressure of about 1-2 bar.
 6. A process according to claim 1, whereinstep (a) is performed at a pH of about 6.5-7.0.
 7. A process accordingto claim 1, wherein the aqueous NH₄F solution has a concentration of30-60% by weight.
 8. A process according to claim 1, wherein the aqueousNH₄F solution has a concentration of about 45% by weight.
 9. A processaccording to claim 1, wherein the thermal hydrolysis reaction (d) isperformed in two reactors.
 10. A process according to claim 9, whereinthe first reactor is maintained at a temperature of up to 350° C.
 11. Aprocess according to claim 9, wherein the first reactor is maintained ata temperature of up to 300-350° C.
 12. A process according to claim 9,wherein the second reactor is maintained at a temperature of up to 900°C.
 13. A process according to claim 9, wherein the second reactor ismaintained at a temperature of up to 800-900° C.
 14. A process accordingto claim 9, wherein the body of the first and/or second reactor is madeof a chromium-nickel alloy.
 15. A process according to claim 9, whereinthe internal surface of the first reactor is made of magnesium or agraphite-reinforced polymer or vitreous carbon.
 16. A process accordingto claim 9, wherein the internal surface of the second reactor is madeof silica.
 17. A process according to claim 1, wherein the aqueousdispersion obtained from the hydrolysis reaction (c) is filtered beforethe thermal hydrolysis reaction (d).
 18. A process according to claim 1,wherein the sludge fraction of step (b) is subjected to a thermalhydrolysis reaction.
 19. A process according to claim 18, wherein saidthermal hydrolysis is performed at a temperature of up to 300-350° C.20. A process according to claim 18, wherein the sludge fraction of step(b) is dehydrated and dried before being subjected to said thermalhydrolysis.
 21. A process according to claim 1, wherein the titanium orecontaining iron is ilmenite.